To date, as an example of the pipe coupling device described above, a pipe coupling device disclosed in International Publication WO2015/087802 (FIG. 10C) has been known, for example. In this pipe coupling device 1′, as shown in FIGS. 12 and 13, for example, a ring-shaped retaining ring 4′ is provided with flange parts 41′ protruding from the outer peripheral surface thereof, and the flange parts 41′ are housed in a recessed part 51′ formed in a pressing plate 5′. Further, coupling members 52′, each having a through-hole 53′ through which a bolt 6′ is inserted, are provided at opposed ends of the recessed part 51′.